An Overview of JavaScript Promises

Category: APIs, JavaScript, modernjs, modernjs-hub, Promises, Raw Javascript

This tutorial covers the basics of JavaScript promises, showing how you can leverage them in your JavaScript development.

The concept of promises is not new to web development. Many of us have already used promises in the form of libraries such as Q, when.js, RSVP.js, etc. Even jQuery has something called a Deferred object, which is similar to a promise. But now we have native support for promises in JavaScript, which is really exciting.

Overview

A Promise object represents a value that may not be available yet, but will be resolved at some point in the future. It allows you to write asynchronous code in a more synchronous fashion. For example, if you use the promise API to make an asynchronous call to a remote web service, you will create a Promise object which represents the data that will be returned by the web service in future. The caveat is that the actual data isn’t available yet. It will become available when the request completes and a response comes back from the web service. In the meantime, the Promise object acts like a proxy to the actual data. Furthermore, you can attach callbacks to the Promise object, which will be called once the actual data is available.

The API

To get started, let’s examine the following code, which creates a new Promise object:

const promise = new Promise((resolve, reject) => {
  //asynchronous code goes here
});

We start by instantiating a new Promise object and passing it a callback function. The callback takes two arguments, resolve and reject, which are both functions. All your asynchronous code goes inside that callback. If everything is successful, the promise is fulfilled by calling resolve(). In case of an error, reject() is called with an Error object. This indicates that the promise is rejected.

Now let’s build something simple which shows how promises are used. The following code makes an asynchronous request to a web service that returns a random joke in JSON format. Let’s examine how promises are used here:

const promise = new Promise((resolve, reject) => {
  const request = new XMLHttpRequest();

  request.open('GET', 'https://api.icndb.com/jokes/random');
  request.onload = () => {
    if (request.status === 200) {
      resolve(request.response); // we got data here, so resolve the Promise
    } else {
      reject(Error(request.statusText)); // status is not 200 OK, so reject
    }
  };

  request.onerror = () => {
    reject(Error('Error fetching data.')); // error occurred, reject the  Promise
  };

  request.send(); // send the request
});

console.log('Asynchronous request made.');

promise.then((data) => {
  console.log('Got data! Promise fulfilled.');
  document.body.textContent = JSON.parse(data).value.joke;
}, (error) => {
  console.log('Promise rejected.');
  console.log(error.message);
});

In the previous code, the Promise constructor callback contains the asynchronous code used to get data the from remote service. Here, we just create an Ajax request to https://api.icndb.com/jokes/random, which returns a random joke. When a JSON response is received from the remote server, it’s passed to resolve(). In case of any error, reject() is called with an Error object.

When we instantiate a Promise object, we get a proxy to the data that will be available in future. In our case, we’re expecting some data to be returned from the remote service at some point in future. So, how do we know when the data becomes available? This is where the Promise.then() function is used. This function takes two arguments: a success callback and a failure callback. These callbacks are called when the Promise is settled (i.e. either fulfilled or rejected). If the promise was fulfilled, the success callback will be fired with the actual data you passed to resolve(). If the promise was rejected, the failure callback will be called. Whatever you passed to reject() will be passed as an argument to this callback.

Try this CodePen example. To view a new random joke, hit the RERUN button in the bottom right-hand corner of the embed. Also, open up your browser console so that you can see the order in which the different parts of the code are executed.

See the Pen An Overview of JavaScript Promises by SitePoint (@SitePoint) on CodePen.

Note that a promise can have three states:

pending (not fulfilled or rejected)
fulfilled
rejected

The Promise.status property, which is code-inaccessible and private, gives information about these states. Once a promise is rejected or fulfilled, this status gets permanently associated with it. This means a promise can succeed or fail only once. If the promise has already been fulfilled and later you attach a then() to it with two callbacks, the success callback will be correctly called. So, in the world of promises, we’re not interested in knowing when the promise is settled. We’re only concerned with the final outcome of the promise.

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